As an image recording method of forming an image on a recording medium, for example, paper, based on image data signals, an electrophotographic system, a sublimation or melting thermal transfer system and an inkjet system have been hitherto known. Among these systems, the electrophotographic system is a complex system and an apparatus therefor is expensive because it requires such a process that an electrostatic latent image is formed on a photoreceptor drum through charge and exposure. The thermal transfer system involves a high running cost and a problem of formation of waste materials due to the use of an ink ribbon, although an apparatus therefor itself is inexpensive in comparison with the electrophotographic system. In the inkjet system, on the other hand, image formation is carried out with an inexpensive apparatus in such a manner that an ink is directly ejected to only a necessary image area on a recording medium, and thus the ink can be used efficiently to reduce the running cost. Further, the inkjet system causes less noise, and thus it is excellent as the image recording method.
The inkjet recording system includes, for example, a system of flying ink droplets by pressure of vapor generated by heat from a heat generator, a system of flying ink droplets by mechanical pressure pulses generated by a piezoelectric element, and a system of flying ink droplets containing charged particles by utilizing an electrostatic field (refer to Patent Documents 1 and 2). The system of flying ink droplets with vapor or mechanical pressure cannot control a flying direction of ink droplet, and sometimes a case may occur where ink droplet is difficult to be accurately reached to the desired position on a printing medium due to distortion of ink nozzle and air convection.
On the contrary, the system utilizing an electrostatic field controls the flying direction of ink droplet with the electrostatic field to enable ink droplet to be accurately reached the desired position, and thus it is advantageous in that an imaged material (printed material) with high image quality can be produced.
As an ink composition for use in the inkjet recording system utilizing an electrostatic field, an ink composition comprising a dispersion medium and charged particles containing at least a colorant is ordinarily employed (refer to Patent Documents 3 and 4). The ink composition containing a colorant can form inks of four colors, i.e., yellow, magenta, cyan and black, by changing the colorant, and can also form special color inks of gold and silver. Accordingly, the ink composition is useful for producing a color imaged material (printed material). In order to produce stably color imaged materials (printed materials) while maintaining high speed and high image quality, however, it is necessary that electrophoretic speed of ink particles is so high that concentration of ink particles sufficiently occurs at an ejection part of an inkjet recording device. For such a purpose, it is confirmed that ink particles must be provided with a sufficient amount of charge. However, liquid developers comprising toner particles having a particle size of 0.5 μm or less ordinarily used in the electrophotographic system are not suitable for the electrostatic inkjet system. Although it is possible to increase the amount of charge of ink particles by increasing the particle size, a satisfactory level for using in the electrostatic inkjet system cannot be achieved at present. Further, when the particle size increases, the amount of charge of ink particles is apt to change with the lapse of time. Thus, it is difficult to obtain an ink composition satisfactorily and stably used in the electrostatic inkjet system.    Patent Document 1: Japanese Patent 3,315,334    Patent Document 2: U.S. Pat. No. 6,158,844    Patent Document 3: JP-A-8-291267 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)